It's not so much that He could commit evil, if you ask me, as that whatever He
could choose to do would by definition not be evil. However, on the receiving
end of the action is where things get tricky.
That's the beginning of a comment left by Shakespeare's Cobbler on my first post in the evil series. I won't quote it in its entirety because he does those 50-word sentences that only I read (because I love him :)).
It reminded me of something I'd read before during a discussion of whether the unbaptized can be saved: We are bound by the sacraments. God is not.
To use a more generally-applicable concept (because some of my small pool of readers are not Catholic): We are bound by the commandments. God is not.
Thou shalt not kill, He said--shortly after sending the Tenth Plague upon Egypt.
The Catholic Church teaches that abortion is wrong, in-vitro fertilization is wrong (see, the Pope doesn't want everyone to have 14 kids so that Catholics can take over the world.), murder is wrong, suicide is wrong, euthanasia is wrong. (This list is not meant to be exhaustive, just illustrative.) Why? Not so we can take over the world (As I already mentioned). Not so we can ruin everybody's fun. There are things the Church forbids simply because she acknowledges the fact that we do not have power over life and death.
God does. God can allow all kinds of things that seem to us to be unspeakably evil. Maybe they are not things that ought to have happened (so we blame Him--"the woman you put here with me..."), but that doesn't mean that God is either evil or powerless. Maybe our world really is shattered beyond repair, shards of blue and green glass lying beyond repair at the bottom of the universe, and from our perspective all is ruined.
Perhaps from God's perspective those little shards are simply the material He needs in order to make something beautiful.